Optical modulator module

ABSTRACT

Disclosed is an optical modulator module including an optical modulator configured to have a signal electrode and a ground electrode; a conductive package configured to accommodate the optical modulator and have electrical continuity with the ground electrode of the optical modulator; a substrate configured to have a ground electrode on a first surface thereof electrically connected to the package by solder or a conductive adhesive and have a signal electrode on another surface thereof; and a lead pin configured to electrically connect the signal electrode of the optical modulator to the signal electrode of the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority ofJapanese Patent Application No. 2010-192142, filed on Aug. 30, 2010, theentire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to an optical modulatormodule.

BACKGROUND

Optical waveguide devices each using an electro-optic crystal such as aLiNbO₃ (LN) substrate and a LiTaO₂ substrate have been developed. Inorder to form such an optical waveguide device, an optical waveguide isfirst formed in such a manner that a metal film made of titanium or thelike is formed and thermally diffused on a part of a crystal substrateor is subjected to proton exchange under benzoic acid after beingpatterned. With provision of electrodes near the optical waveguide, theoptical waveguide device is formed. An example of such an opticalwaveguide device includes an optical modulator.

An optical modulator is accommodated in a metal package and mounted in atransmitter as an optical modulator module. Recently, varioussurface-mounting type components have been developed for the purpose ofimproving mounting performance (see, for example, Patent Document 1).However, they give rise to a problem in high-frequency characteristics.

Patent Document 2 discloses a configuration in which a spacer isinterposed between a package and a flexible substrate to alleviateimpedance mismatching. Patent Document 3 discloses a configuration inwhich a lead pin is mounted parallel on the signal electrode pad of anFPC and solder-connected.

-   Patent Document 1: Japanese Laid-open Patent Publication No.    4-336702-   Patent Document 2: Japanese Laid-open Patent Publication No.    2007-42756-   Patent Document 1: Japanese Laid-open Patent Publication No.    2009-252918

However, the configurations of Patent Documents 2 and 3 put limitationson space.

SUMMARY

According to an aspect of the present invention, there is provided anoptical modulator module including an optical modulator configured tohave a signal electrode and a ground electrode; a conductive packageconfigured to accommodate the optical modulator and have electricalcontinuity with the ground electrode of the optical modulator; asubstrate configured to have a ground electrode on a first surfacethereof electrically connected to the package by solder or a conductiveadhesive and have a signal electrode on another surface thereof; and alead pin configured to electrically connect the signal electrode of theoptical modulator to the signal electrode of the substrate.

The object and advantages of the present invention will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the present invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a schematic plan view of a Mach-Zehnder type opticalmodulator and a cross-sectional view taken along line A-A in FIG. 1A,respectively;

FIGS. 2A, 2B, and 2C are a schematic plan view of an optical modulatormodule according to a first comparative example, a cross-sectional viewof the optical modulator module taken along line B-B in FIG. 2A, and aview illustrating a state in which a ground electrode is connected to aground electrode formed at the under surface of a relay substrate by wayof a via-hole, respectively;

FIGS. 3A and 3B are view for explaining an optical modulator moduleaccording to a second comparative example;

FIGS. 4A through 4D are views for explaining an optical modulator moduleaccording to a first embodiment;

FIGS. 5A and 5B are graphs illustrating calculation results of arelationship between S parameters at 30 GHz and a shortest distance dY;

FIGS. 6A and 6B are views for explaining another example of a flexiblesubstrate according to the first embodiment;

FIGS. 7A and 7B are views for explaining another example of the flexiblesubstrate;

FIGS. 8A and 8B are views for explaining another example of the flexiblesubstrate;

FIGS. 9A and 9B are views for explaining another example of the flexiblesubstrate;

FIG. 10 is a view for explaining another example of a package;

FIGS. 11A through 11C are view for explaining an optical modulatormodule according to a second embodiment;

FIGS. 12A through 12D are views for explaining another example of aflexible substrate;

FIGS. 13A through 13C are views for explaining another example of theflexible substrate;

FIGS. 14A and 14B are views for explaining another example of theflexible substrate;

FIGS. 15A and 15B are views for explaining connection between theflexible substrate and a print substrate;

FIGS. 16A and 16B are graphs for explaining calculation results of Sparameters;

FIGS. 17A through 17C are views for explaining an optical modulatormodule according to a third embodiment;

FIG. 18 is a view for explaining the optical modulator module accordingto the third embodiment; and

FIG. 19 is a block diagram for explaining the entire configuration of anoptical transmitter according to a fourth embodiment.

DESCRIPTION OF EMBODIMENTS

Next, embodiments are described below with reference to the accompanyingdrawings.

First, a Mach-Zehnder type optical modulator is described as an exampleof an optical modulator provided in an optical modulator module. FIG. 1Ais a schematic plan view of a Mach-Zehnder type optical modulator 10.FIG. 1B is a cross-sectional view taken along line A-A in FIG. 1A. Asillustrated in FIGS. 1A and 1B, the optical modulator 10 has a substrate14 in which an optical waveguide is formed. The substrate 14 is anelectro-optic substrate having an electro-optic crystal such as a LiNbO₃(LN) substrate and a LiTaO₂ substrate.

The optical waveguide includes an incident waveguide, parallelwaveguides 11a and 11b formed to branch from the incident waveguide, andan emitting waveguide in which the parallel waveguides 11a and 11b mergewith each other. The optical waveguide is formed in such a manner thatmetal such as Ti (titanium) is thermally-diffused into the substrate 14.

As illustrated in FIG. 1B, a buffer layer 15 is provided at a surface onthe side of the optical waveguide of the substrate 14. The opticalwaveguide is covered with the buffer layer 15. The buffer layer 15 isprovided to prevent light transmitted through the optical waveguide frombeing absorbed in electrodes described below. The buffer layer 15 ismade of, for example, SiO₂ or the like and has a thickness of about 0.2through 2 μm.

On the parallel waveguide 11b, a signal electrode 12 is provided via thebuffer layer 15. On the parallel waveguide 11a, a ground electrode 13bis provided via the buffer layer 15. Further, on the buffer layer 15, aground electrode 13a is provided on the side opposite to the groundelectrode 13b in such a manner as to sandwich the signal electrode 12between the ground electrodes 13a and 13b. Thus, the signal electrode 12and the ground electrodes 13a and 13b form coplanar electrodes. If aZ-cut substrate is used as the substrate 14, the signal electrode 12 andthe ground electrode 13b are arranged right above the parallelwaveguides to make use of refractive-index fluctuations resulting fromelectrolysis in a Z direction.

In order to drive the optical modulator 10 at high speed, the ends ofthe signal electrode 12 and the ground electrodes 13a and 13b areconnected by a resistor to form a traveling wave electrode and a microwave signal is applied on the input side of the traveling waveelectrode. In this case, the refractive indexes of the parallelwaveguides 11a and 11b fluctuate, for example, like +Δn and −Δn due toan electric field. Thus, due to fluctuations in phase difference betweenthe parallel waveguides 11a and 11b, Mach-Zehnder interference occurs.As a result, signal light having modulated intensity is output from theemitting waveguide. The effective refractive index of microwaves can becontrolled with a change in the cross-sectional shapes of theelectrodes, and high-speed light response characteristics can beobtained by matching the speeds of light and microwaves.

FIG. 2A is a schematic plan view of an optical modulator moduleaccording to a first comparative example. FIG. 2B is a cross-sectionalview of the optical modulator module taken along line B-B in FIG. 2A. Asillustrated in FIGS. 2A and 2B, the optical modulator 10 is accommodatedin a metal package 20. Although not illustrated in FIGS. 2A and 2B, acover may be provided above the package 20. At one end of the package 20is provided a connector 22a in which an optical fiber 21a penetrates. Atthe other end of the package 20 is provided a connector 22b in which anoptical fiber 21b penetrates. The incident waveguide of the opticalmodulator 10 is arranged to coincide with the optical axis of theoptical fiber 21a. The emitting waveguide of the optical modulator 10 isarranged to coincide with the optical axis of the optical fiber 21b.

An end of the signal electrode 12 and first ends of the groundelectrodes 13a and 13b are connected to each other via a terminalresistor 23. Another end of the signal electrode 12 and other ends ofthe ground electrodes 13a and 13b are guided to an outside via a relaysubstrate 31. At the top surface of the relay substrate 31, a signalelectrode 33 for the signal electrode 12 is formed. To the signalelectrode 33 is connected a lead pin 36 by solder 34. The lead pin 36extends to the outside via a coaxial connector 35 that penetrates theside wall of the package 20. Note that since the lead pin 36 is harderthan lead wire, it can accurately maintain an interval between groundsuch as the metal package 20 and the lead pin 36. Accordingly, the leadpin 36 can accurately take impedance matching as a transmission path.

At the upper surface of the relay substrate 31, a ground electrode 33aconnected to the ground electrodes 13a and 13b is further formed. Asillustrated in FIG. 2C, the ground electrode 33a is connected to aground electrode 32 formed at the under surface of the relay substrate31 by way of a via-hole 33b. The ground electrode 32 has electricalcontinuity with the package 20. The package 20 is grounded.

In the optical modulator module according to the first comparativeexample, it is necessary to input an electric signal output from adriver amplifier to the lead pin 36 via an edge-mount type connector orthe like. Accordingly, it is difficult for the optical modulator moduleto be mounted.

FIGS. 3A and 3B are views for explaining an optical modulator moduleaccording to a second comparative example. The optical modulator moduleaccording to the second comparative example is a surface-mounting typemodule for facilitating its mounting. In the optical modulator moduleaccording to the second comparative example, an electric signal outputfrom a driver amplifier is input from a print substrate. Accordingly,the optical modulator module has improved mounting performance.

FIG. 3A is a view corresponding to the view illustrated in FIG. 2B. Inthe second comparative example, a flexible substrate 41 is used for thepurpose of improving mounting performance. The flexible substrate 41having flexibility is made of polyimide, liquid crystal polymer, or thelike. In this example, an insulative glass member 25 that penetrates apackage 20 is provided at the under surface of the package 20. The glassmember 25 is formed into a cylindrical shape as an example. Further, theflexible substrate 41 is provided at the under surface of the glasssubstrate 25. A lead pin 36 extends to the under surface of the flexiblesubstrate 41 while penetrating the glass substrate 25 and the flexiblesubstrate 41.

In this example, a lead pin 37a (FIG. 3B) connected to a groundelectrode 45a via a ground electrode 32 and a lead pin 37b connected toa ground electrode 45b via the ground electrode 32 are provided. Thelead pins 37a and 37b extend to the under surface of the flexiblesubstrate 41 while penetrating the glass member 25 and the flexiblesubstrate 41. The lead pins 37a and 37b are symmetrically arranged aboutthe lead pin 36.

FIG. 3B is a view seen from the under surface side of the flexiblesubstrate 41. As illustrated in FIG. 3B, a signal electrode 43 and theground electrodes 45a and 45b are formed at the under surface of theflexible substrate 41. The lead pin 36 is connected to the signalelectrode 43 via solder 42. The lead pin 37a is connected to the groundelectrode 45a via solder 44a. The lead pin 37b is connected to theground electrode 45b via solder 44b. The ground electrodes 45a and 45bare symmetrically arranged about the signal electrode 43. Thus, acoplanar line (CPW) is formed.

According to the configuration of the second comparative example, sincethe ground lead pins and the signal lead pin can be soldered at theunder surface of the flexible substrate 41, the optical modulator modulehas high mounting performance. This configuration is particularlyeffective if the optical modulator module has a large number of the leadpins. However, it is necessary to set an interval of, for example, 1 mmor more between the adjacent lead pins in order to solder the lead pins.In this case, the interval between the signal electrode and the groundelectrodes becomes large. Thus, an characteristic impedance locallygreatly deviates from a desired value (for example 50Ω), wherebyreflecting characteristics (FIG. 5A, S11) are degraded. Further, sincecontact areas between the ground lead pins and the electrodes arelimited to only the parts of the lead pins, grounding cannot besufficiently established for high frequency. Thus, transmittingcharacteristics (S21) are degraded. The reflecting characteristics (S11)and the transmitting characteristics (S21) may present problems at ahigh-speed modulation band such as 20 Gbps and 40 Gbps. Further, amodulator having plural signal lines such as a DQPSK modulator and aDP-QPSK modulator may give rise to problems in that it has a difficultyin its high density and requires a large mounting space. In view ofthis, the following embodiments describe optical modulator modulescapable of realizing both high frequency characteristics and mountingperformance while accommodating limitations in space.

Here, the reflecting characteristics (S11) refer to the ratio ofreflecting power (Pr) to input power Pin input from a driver amplifierto an optical modulator. The transmitting characteristics (S21) refer tothe ratio of output power Pout to the input power Pin input from thedriver amplifier to the optical modulator. Specifically, the reflectingcharacteristics (S11) are calculated by Pr/Pin (dB), and thetransmitting characteristics (S21) are calculated by Pout/Pin (dB).

First Embodiment

FIGS. 4A through 4C are views for explaining an optical modulator module100 according to a first embodiment. The optical modulator module 100 isa surface-mounting type module for facilitating its mounting. FIG. 4A isa view corresponding to the view illustrated in FIG. 3A. FIG. 4B is aview corresponding to the view illustrated in FIG. 3B and seen from theunder surface sides of a flexible substrate 41 and a package 20. FIG. 4Cis a view seen from the top surface side of the flexible substrate 41.

As illustrated in FIGS. 4A and 4B, a lead pin 36 penetrates a glassmember 25 and extends to the under surface of the flexible substrate 41while being in contact with the side surface of the flexible substrate41. In this embodiment, the glass member 25 is formed into a cylindricalshape, and the lead pin 36 penetrates the substantial center of thecylindrical shape. The lead pin 36 is connected to a signal electrode 43at the under surface of the flexible substrate 41 via solder 42.

Note that since the lead pin 36 is provided to be in contact with theside surface of the flexible substrate 41, a contact area between thelead pin 36 and the flexible substrate 41 may not be sufficientlyobtained. However, if the cross section of the lead pin 36 is formedinto a rectangular shape, it is possible to sufficiently ensure thecontact area between the lead pin 36 and the flexible substrate 41.

On the top surface of the flexible substrate 41, a ground electrode 45having a predetermined width is formed. Since the lead pin 36 isprovided along the side surface of the flexible substrate 41, the groundelectrode 45 is formed to be away from the side surface. For example,the ground electrode 45 may be provided to avoid a semi-circular regionsurrounding a part at which the lead pin 36 is provided. The groundelectrode 45 is connected to the external wall (for example, the undersurface) of the package 20 via solder 44. For example, the groundelectrode 45 may be connected to the under surface of the package 20 ata part adjacent to the glass member 25.

In this embodiment, with the provision of the lead pin 36 at the sidesurface of the flexible substrate 41, the side surface of the flexiblesubstrate 41 is away from the package 20. This enables confirmation asto whether the solder 44 flows out from the under surface side of theflexible substrate 41. In FIG. 4B, flowing out of the solder isconfirmed by the existence of solder 44a and 44b, which enables theconfirmation of the connecting state between the package 20 and theground electrode 45 of the flexible substrate 41. Accordingly,manufacturing yield can be maintained at high level.

In this embodiment, without the use of a ground lead pin, the groundelectrode 45 formed on the top surface of the flexible substrate 41 andthe external wall of the package 20 are connected to each other. In thiscase, a contact area between the ground electrode 45 and the externalwall of the package 20 become larger compared with a case in which theground lead pin is used. Thus, grounding can be sufficiently establishedfor high frequency. As a result, degradation of S parameters can besuppressed. Further, since there is no need to use a spacer or the liketo reduce impedance mismatching, limitation in space can be suppressed.

Further, in this embodiment, a micro strip line (MSL) structure isformed by the ground electrode 45 having a predetermined width on thetop surface of the flexible substrate 41 and the signal electrode 43 onthe under surface of the flexible substrate 41. A characteristicimpedance is controlled by the influences of the thickness of asubstrate and a signal line width. Therefore, controlling the thicknessof the flexible substrate 41 and the line width of the signal electrode43 in the vicinity of their desired values provides an impedance havinga desired value (for example, 50Ω). Thus, the optical modulator module100 has improved reflecting characteristics (S11).

Further, the provision of the signal electrode 43 on the under surfaceof the flexible substrate 41 facilitates the mounting of the opticalmodulator module 100. Note that the ground electrode 45 can extend tothe under surface of the flexible substrate 41 via a via-hole or thelike. Accordingly, the optical modulator module 100 can besurface-mounted by the flexible substrate 41.

Thus, according to this embodiment, the optical modulator module 100 canrealize its high frequency characteristics and mounting performancewhile accommodating limitations in space.

Note that the solder 44 used in this embodiment may be replaced by aconductive adhesive or the like. Further, as illustrated in FIG. 4D, ina case where a lead pin 36 having a circular cross section is used, apart of the flexible substrate 41 at which the lead pin 36 is in contactwith may be cut into a semicircular shape. In this case, a contact areabetween the lead pin 36 and the flexible substrate 41 can be increased.Moreover, the flexible substrate 41 used in this embodiment may bereplaced by a substrate having high rigidity.

Note that although a cross-sectional shape at a connection part betweenthe package 20 and the flexible substrate 41 preferably has a MSLstructure, a part of the ground electrode 45 is formed to avoid the leadpin 36 as illustrated in FIG. 4C. This is aimed at avoidingshort-circuits and local reduction of impedance. However, if the leadpin 36 is excessively away from the ground electrode 45, a MSL mode isnot established and high frequency characteristics are degraded.Therefore, a shortest distance dY between the ground electrode 45 andthe lead pin 36 is preferably set to be within an appropriate range.FIG. 5A is a graph illustrating calculation results of a relationshipbetween the S parameters at 30 GHz and the shortest distance dY. Asillustrated in FIG. 5A, the shortest distance dY is preferably set to beless than or equal to 260 μm in order to suppress the reflectingcharacteristics (S11) below −20 dB.

Note that in the second comparative example, the signal lead pin and theground lead pins penetrate and protrude from the substrate. However, inthis embodiment, only the lead pin 36 protrudes downward from the undersurface of the flexible substrate 41. Thus, a distance from the tip endof the lead pin 36 to the ground electrode 45 becomes large. If theprotruding length of the lead pin 36 becomes large, impedancemismatching may become significant. In view of this, a relationshipbetween the protruding length of the lead pin 36 and the S parameters at30 GHz was calculated. The calculation results are illustrated in FIG.5B. As illustrated in FIG. 5B, in order to suppress the reflectingcharacteristics (S11) below −20 dB, the length of the lead pin 36protruding from the flexible substrate 41 is preferably set to be lessthan or equal to 590 μm.

According to the configuration illustrated in FIGS. 4A through 4C, thecontact area between the ground electrode 45 and the package 20 becomeslarge. In this case, when temperature or humidity changes, stressresulting from a difference in expansion coefficient between theflexible substrate 41, the solder 44, and the package 20 becomes high,which may bring about characteristic degradation or breakage in themodulator. This problem becomes remarkable particularly in a modulatorhaving a large number of terminals such as a DP-QPSK modulator and maybecome a main factor that degrades the long-term reliability of themodulator.

(Another Example (1) of Flexible Substrate)

Therefore, it is preferable that the ground electrode 45 can be observedfrom the under surface side of the flexible substrate 41. FIGS. 6A and6B are views for explaining another example of the flexible substrate41. FIG. 6A is a perspective view illustrating the under surface of theflexible substrate 41 and the side surface of the flexible substrate 41on the side of the lead pin 36. FIG. 6B is a plan view illustrating theunder surface of the flexible substrate 41.

As illustrated in FIGS. 6A and 6B, a notch is formed in the flexiblesubstrate 41 at a part at which the lead pin 36 is arranged. The leadpin 36 is arranged at the notch and connected to the signal electrode43. In the side surface of the flexible substrate 41 on the side of thelead pin 36, notches 46a and 46b may be further formed. The shapes ofthe notches 46a and 46b include, but are not particularly limited to, asemi-circular shape as an example. The notches 46a and 46b aresymmetrically provided about the notch at which the lead pin 36 isarranged. Further, the notches 46a and 46b are provided at a part atwhich the ground electrode 45 illustrated in FIG. 4C is formed. Thus,the ground electrode 45 can be confirmed from the under surface side ofthe flexible substrate 41. Further, flowing out of the solder 44 thatconnects the ground electrode 45 to the package 20 can be confirmed.

(Another Example (2) of Flexible Substrate)

FIGS. 7A and 7B are views for explaining another example of the flexiblesubstrate 41. FIG. 7A is a perspective view illustrating the undersurface of the flexible substrate 41 and the side surface 41 on the sideof the lead pin 36 of the flexible substrate 41. FIG. 7B is a plan viewillustrating the under surface of the flexible substrate 41.

As illustrated in FIGS. 7A and 7B, a part of the ground electrode 45 maybe a flying lead that protrudes from the end part of the flexiblesubstrate 41. In this case, the flying lead can be confirmed from theend of the flexible substrate on the side of the lead pin 36. Thus, theground electrode 45 can be confirmed. Further, flowing out of the solder44 that connects the ground electrode 45 to the package 20 can beconfirmed.

(Another Example (3) of Flexible Substrate)

FIGS. 8A and 8B are views for explaining another example of the flexiblesubstrate 41. FIG. 8A is a perspective view illustrating the undersurface of the flexible substrate 41 and the side surface of theflexible substrate 41 on the side of the lead pin 36. FIG. 8B is a planview illustrating the under surface of the flexible substrate 41. Asillustrated in FIGS. 8A and 8B, a part of the ground electrode 45 mayextend to the side surface of the flexible substrate 41. Thus, theground electrode 45 can be confirmed. Further, flowing out of the solder44 that connects the ground electrode 45 to the package 20 can beconfirmed.

(Another Example (4) of Flexible Substrate)

FIGS. 9A and 9B are views for explaining another example of the flexiblesubstrate 41. FIG. 9A is a perspective view illustrating the undersurface of the flexible substrate 41 and the side surface of theflexible substrate 41 on the side of the lead pin 36. FIG. 9B is a planview illustrating the under surface of the flexible substrate 41. Asillustrated in FIGS. 9A and 9B, electrodes 47a and 47b may be formed oneon each side of a notch at the side surface of the lead pin 36.Moreover, the ground electrode 45 may be connected to the electrodes 47aand 47b. In this case, the ground electrode 45 can be confirmed.Further, flowing out of the solder 44 that connects the ground electrode45 to the package 20 can be confirmed.

(Another Example of Package)

FIG. 10 is a view for explaining another example of the package 20. FIG.10 is a perspective view illustrating the under surfaces of the flexiblesubstrate 41 and the package 20 and the side surface of the flexiblesubstrate on the side of the lead pin 36. As illustrated in FIG. 10,grooves 26a and 26b may be formed in the package 20 at a part at whichthe package 20 is connected to the ground electrode 45. That is, at theunder surface of the package 20, a concave part may be formed at thepart at which the package 20 is connected to the ground electrode 45.

In this case, when the package 20 is connected to the ground electrode45 by the solder 44, the solder 44 flows in the grooves 26a and 26b.Thus, flowing out of the solder 44 can be confirmed. It is preferablethat the grooves 26a and 26b be provided to cross the end part of theflexible substrate 41 on the side of the lead pin 36. This is because itfacilitates the confirmation of the flowing out of the solder 44.

Second Embodiment

FIGS. 11A through 11C are views for explaining an optical modulatormodule 100a according to a second embodiment. FIG. 11A is a viewcorresponding to the view illustrated in FIG. 4A. FIG. 11B is a viewcorresponding to the view illustrated in FIG. 4B and seen from the undersurface side of a flexible substrate 41. FIG. 11C is a viewcorresponding to the view illustrated in FIG. 4C and seen from the topsurface of the flexible substrate 41.

As illustrated in FIGS. 11A through 11C, a lead pin 36 penetrates aglass member 25 and extends to the lower surface of the flexiblesubstrate 41 while penetrating the flexible substrate 41. In thisembodiment, the glass member 25 is formed into a cylindrical shape, andthe lead pin 36 penetrates the substantial center of the cylindricalshape. The lead pin 36 is connected to a signal electrode 43 at theunder surface of the flexible substrate 41 by solder 42. As illustratedin FIG. 11C, a ground electrode 45 is formed to have a predetermineddistance between the ground electrode 45 and a part of the flexiblesubstrate 41 at which the lead pin 36 penetrates. Thus, a short circuitof the ground electrode 45 and the lead pin 36 can be prevented.

In this embodiment, since the lead pin 36 penetrates the flexiblesubstrate 41, the flexible substrate 41 can be connected to a package 20to surround the glass member 25. In this case, a contact area betweenthe flexible substrate 41 and the package 20 is increased. Thus,adhesion between the flexible substrate 41 and the package 20 can beimproved.

Note that a gap is preferably formed between the flexible substrate 41and the package 20 on the extension of the flexible substrate 41. Inthis case, the connection part of the ground electrode 45 is exposed atthe gap. Thus, the connection part of the ground electrode 45 can beconfirmed from the under surface side of the flexible substrate 41.Further, at a connection part between the flexible substrate 41 and thepackage 20, a notch is preferably formed in the package 20 to expose theground electrode 45. In this case, the connection part of the groundelectrode 45 can be confirmed.

Further, on the top surface of the ground electrode 45, an insulativecoverlay 48 is preferably provided between the solder 44 and the leadpin 36. In this case, a short circuit of the lead pin 36 and the groundelectrode 45 due to flowing out of the solder 44 is suppressed. As thecoverlay 48, polyimide or the like can be used.

FIGS. 12A through 12D are views for explaining an example of theflexible substrate 41 according to this embodiment. FIGS. 12A through12D are views seen from the under surface side of the flexible substrate41. As illustrated in FIG. 12A, through-holes 49 may be formed in theflexible substrate 41 at apart at which the ground electrode 45 isformed. The through-holes 49 are formed to avoid the glass member 25.

As illustrated in FIG. 12B, notches 49a may be formed in the flexiblesubstrate 41 at the part at which the ground electrode 45 is formed. Thenotches 49a are formed to avoid the glass member 25. As illustrated inFIG. 12C, plural through-holes 49b may be formed in the flexiblesubstrate 41 to surround the glass member 25. The through-holes 49b maybe the same in shape as the through-hole of the lead pin 36. In thiscase, a through-hole forming process is simplified.

Note that in consideration of high frequency characteristics, solder ispreferably attached on the side of the lead pin 36 at the outer edge ofthe through-holes 49 and 49b and the notches 49a. Further, in order toreduce stress, the solder is preferably not attached on the side farfrom the lead pin 36 at the outer edge of the through-holes 49 and 49band the notches 49a. Moreover, the through-holes 49 and 49b and thenotches 49a are preferably covered with a transparent dielectric layer.In this case, climbing of the solder can be suppressed.

As illustrated in FIG. 12D, vias 49c may be formed in the under surfaceof the flexible substrate 41 on the side opposite to the signalelectrode 43 about the lead pin 36. In this case, straight traveling ofa signal capable of being not linked to the lead pin can be suppressed.Further, a ground electrode 50 may be provided on the under surface ofthe flexible substrate 41 to be connected to the ground electrode 45 byway of the vias 49c. In this case, since a grounded coplanar structureis formed by the signal electrode 43, the ground electrode 45, and theground electrode 50, grounding on the periphery of the lead pin 36 isenhanced. Note that the vias 49c may be an embedded, but through-typevias can realize the enhancement of grounding and the confirmation ofthe solder. Note that the configuration illustrated in FIGS. 12A through12D is also applicable to the first embodiment in which the lead pin 36is provided along the side surface of the flexible substrate 41.

In the above respective embodiments, through-holes for fixation to thepackage 20 may be formed in the flexible substrate 41. FIG. 13A is aplan view for explaining an example in which through-holes for fixationare formed in the flexible substrate 41. FIG. 13A is a view illustratingthe under surface of the flexible substrate 41. As illustrated in FIG.13A, the plural through-holes 51 for fixation may be formed in theflexible substrate 41.

FIG. 13B is a plan view for explaining an example in which fixation pins52 are inserted into the through-holes 51. FIG. 13C is a cross-sectionalview taken along line C-C in FIG. 13B. As illustrated in FIGS. 13B and13C, insertion of the fixation pins 52 into the through-holes 51 canimprove fixation strength between the flexible substrate 41 and thepackage 20. Further, with the provision of the fixation pins 52 so as toprotrude more than the lead pin 36 on the under surface side of theflexible substrate 41, not only the productivity of the opticalmodulator module but also fixation strength of the flexible substrate 41can be improved.

Note that in a case where the flexible substrate 41 has a MSL structure,the value of an impedance in a MSL mode that transmits the flexiblesubstrate 41 becomes important. Unlike a CPW mode, the thickness of theflexible substrate 41 and the line width of the signal electrode 43become important parameters. If the flexible substrate 41 is thin whenthe flexible substrate 41 is set to have an electric resistance of 50Ω,it is necessary to reduce the line width of the signal electrode 43. Inthis case, a conductor loss may be increased, and an impedance may begreatly changed due to a slight change in signal line width. Therefore,the thickness of the flexible substrate 41 is preferably in the range ofabout several tens through 100 μm.

However, if the flexible substrate 41 impairs flexibility due to itsincreased thickness, it may have a part having more flexibility than theconnection part at which the flexible substrate 41 is connected to thepackage 20. For example, as illustrated in FIG. 14A, the width of theground electrode 45 on the flexible substrate 41 may be narrowed at apart other than the connection part. Further, as illustrated in FIG.14B, the ground electrode 45 may be formed into a meshed shape at a partother than the connection part. In these cases, flexibility of theflexible substrate 41 can be improved.

Alternatively, the ground electrode 45 may be provided only on the samesurface as the signal electrode 43 at a part other than the connectionpart. For example, as illustrated in FIG. 15A, it is assumed that aconnection part between the flexible substrate 41 and the package 20 isa region 1 3 and a connection part between the flexible substrate 41 anda print substrate 60 is a region 3 1. The flexible substrate 41 isconnected to the print substrate 60 at an end part on the side oppositeto the lead pin 36. It is assumed that a region between the regions 1and 3 is a region 2. Although the regions 1 and 3 have a MSL or GCPW,provision of a single-sided electrode in the region 2 can improveflexibility of the flexible substrate 41.

Note that if the flexible substrate 41 has sufficient flexibility, thelead pin 36 may be provided to be perpendicular to the side surface ofthe package 20 and the flexible substrate 41 may be folded by 90 degreesas illustrated in FIG. 15B.

FIG. 16A is a graph illustrating calculation results of S parameters ofthe optical modulator module according to the second comparativeexample. FIG. 16B is a graph illustrating calculation results of the Sparameters of the optical modulator module 100a according to the secondembodiment illustrated in FIG. 12D. As illustrated in FIGS. 16A and 16B,the reflecting characteristics (S11) and the transmittingcharacteristics (S21) of the optical modulator module 100a according tothe second embodiment are improved compared with the optical modulatormodule according to the second comparative example.

Third Embodiment

FIG. 17A is a view for explaining an optical modulator module 100baccording to a third embodiment. FIG. 17A is the view corresponding tothe view illustrated in FIG. 4A. FIGS. 17B and 17C are views seen fromthe under surface side of a package 20.

In this embodiment, an external conductor 39 is provided on theperiphery of a glass member 25 as illustrated in FIG. 17A. Thus, acoaxial line (having a resistance of, for example, 50Ω) is formed by alead pin 36, the glass member 25, and the external conductor 39. In thisembodiment, the coaxial line is inserted into a concave part at theunder surface of the package 20.

In this case, solder 44 is preferably connected to a flexible substrate41 to encircle the periphery of the external conductor 39. Therefore, agroove is preferably provided along the periphery of the externalconductor 39 in the package 20 as illustrated in FIG. 17B. In this case,the solder 44 encircles the periphery of the external conductor 39 viathe groove. With the provision of an inlet for flowing the solder 44 inthe groove and an outlet for confirming flowing out of the solder 44,soldering can be effectively performed.

Note that the groove encircling the periphery of the external conductor39 may have two paths as illustrated in FIG. 17C. In this case, sincetwo outlets for confirming the flowing out of the solder 44 areprovided, the solder flowing out from both of the paths can beconfirmed. Thus, confirmation as to whether the solder 44 flows in bothof the paths can be made. As a result, sufficient grounding can beobtained, and degradation of the S parameters can be suppressed.

Note that if adhesion between the glass member 25 and the flexiblesubstrate 41 is reduced, a gap is formed between the glass member 25 andthe flexible substrate 41. In this case, a characteristic impedance maybecome large at the gap. Therefore, as illustrated in FIG. 18, thethickness (Wair) of the lead pin 36 at a part at which the lead pin 36is connected to a signal electrode 43 may be greater than the thickness(Wglass) of the lead pin 36 at a part at which the lead pin 36penetrates the glass member 25. In this case, the impedance iscorrected. Note that the cross section of the lead pin 36 at the part atwhich the lead pin 36 is connected to the signal electrode 43 may be acircular shape or a rectangular shape but it is not particularlylimited.

Fourth Embodiment

FIG. 19 is a block diagram for explaining the entire configuration of anoptical transmitter according to a fourth embodiment. As illustrated inFIG. 19, the optical transmitter 200 has an optical device 210, a datageneration unit 220, and the like. The optical device 210 is asemiconductor laser or the like having any one of the optical modulatormodules described above. The data generation unit 220 transmits adriving signal for driving the optical device 210 to the optical device210. The optical device 210 outputs an optical modulation signal inresponse to the driving signal from the data generation unit 220. Theoptical modulation signal is output to an outside via an optical fiberor the like.

Each of the embodiments described above uses the Mach-Zehnder typeoptical modulator module as an optical modulator, but the opticalmodulator is not limited to it. Any optical modulator having a groundelectrode and a signal electrode is applicable to the embodimentsdescribed above.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the presentinvention and the concepts contributed by the inventor to furthering theart, and are to be construed as being without limitation to suchspecifically recited examples and conditions, and the organization ofsuch examples in the specification does not relate to a showing of thesuperiority or inferiority of the present invention. Although theembodiment of the present invention has been described in detail, itshould be understood that the various changes, substitutions, andalterations could be made hereto without departing from the spirit andscope of the present invention.

What is claimed is:
 1. An optical modulator module comprising: anoptical modulator configured to have a first signal electrode and afirst ground electrode; a conductive package configured to accommodatethe optical modulator and have electrical continuity with the firstground electrode of the optical modulator; an insulative memberconfigured to penetrate the conductive package; a substrate configuredto have a second ground electrode on a first surface thereofelectrically connected to the package by solder or a conductive adhesiveand have a second signal electrode on a second surface thereof; and alead pin configured to penetrate the insulative member and electricallyconnect the first signal electrode of the optical modulator to thesecond signal electrode of the substrate, wherein a gap is formedbetween the substrate and the package on an extension of the substrate.2. The optical modulator module according to claim 1, wherein the secondground electrode and the second signal electrode of the substrate have amicro strip line structure.
 3. The optical modulator module according toclaim 1, wherein the lead pin extends to the second surface while beingin contact with a side surface of the substrate.
 4. The opticalmodulator module according to claim 3, wherein the lead pin has arectangular cross section.
 5. The optical modulator module according toclaim 3, wherein a notch at which the lead pin is arranged is formed inthe side surface of the substrate.
 6. The optical modulator moduleaccording to claim 1, wherein a notch for exposing the second groundelectrode of the substrate is formed in a side surface of the substrate.7. The optical modulator module according to claim 1, wherein a notch isformed in a side surface of the substrate, and the second groundelectrode of the substrate extends to the notch.
 8. The opticalmodulator module according to claim 1, wherein a groove crossing an endpart of the substrate is formed in a surface of the package connected tothe substrate.
 9. The optical modulator module according to claim 1,wherein the gap is formed by a notch for exposing the second groundelectrode, and the notch is formed in the package at a connection partbetween the package and the second ground electrode of the substrate.10. The optical modulator module according to claim 1, wherein athrough-hole is formed in the substrate at a connection part between thepackage and the second ground electrode of the substrate.
 11. Theoptical modulator module according to claim 10, wherein a third groundelectrode is formed on the second surface of the substrate, and thethird ground electrode on the second surface is electrically connectedto the second ground electrode on the first surface via thethrough-hole.
 12. The optical modulator module according to claim 1,wherein a notch extending from the first surface of the substrate to thesecond surface thereof is formed in a connection part between thepackage and the second ground electrode of the substrate.
 13. Theoptical modulator module according to claim 1, wherein an insulativecoverlay is provided between the second ground electrode and the leadpin on the first surface of the substrate.
 14. The optical modulatormodule according to claim 1, wherein a shortest distance between thelead pin and the second ground electrode of the substrate is less thanor equal to 260 μm.
 15. The optical modulator module according to claim1, wherein a length of the lead pin protruding from the second surfaceof the substrate is less than or equal to 590 μm.
 16. The opticalmodulator module according to claim 1, wherein a fixation pin for fixingthe substrate to the package via a through-hole is provided in thesubstrate.
 17. The optical modulator module according to claim 1,wherein the substrate is a flexible substrate.
 18. The optical modulatormodule according to claim 17, wherein the substrate has a part havingmore flexibility than a connection part between the substrate and thepackage.
 19. The optical modulator module according to claim 1, whereina thickness of the lead pin at a part protruding from a glass member toan outside of the package is greater than a thickness of the lead pin inthe glass member.
 20. The optical modulator module as claimed in claim1, wherein an end portion of the substrate is positioned lower than anarea where the package is fixed on the substrate, the end portion beingopposite to the area.
 21. The optical modulator module as claimed inclaim 1, wherein the substrate includes a through hole penetrating fromthe first surface to the second surface, and the lead pin penetrates thethrough hole.